1. Field of Invention
The present invention relates to piezoelectric electrical energy transfer devices. More particularly, the present invention relates to a multi-layered piezoelectric transformer whose input and output voltages and currents are isolated from each other, and whose output preserves the input signal frequency in a broad bandwidth.
2. Description of the Prior Art
Wound-type electromagnetic transformers have been used for raising or lowering input voltages (step-up and step-down transformation, respectively) in internal power circuits of devices such as televisions or in charging devices of copier machines which require high voltage, or in circuits requiring a low voltage on the output side, such as in telecommunication circuits. Such electromagnetic transformers take the form of a conductor wound onto a core made of a magnetic substance. Because a large number of turns of the conductor are required to realize high transformation ratios, electromagnetic transformers that are effective, yet at the same time compact and slim in shape are extremely difficult to produce.
To remedy this problem, piezoelectric transformers utilizing the piezoelectric effect have been provided in the prior art. In contrast to the general electromagnetic transformer, the piezoelectric ceramic transformer has a number of advantages. The size of a piezoelectric transformer can be made smaller than electromagnetic transformers of comparable transformation ratio. Piezoelectric transformers can be made nonflammable, and they produce no electromagnetically induced noise.
The ceramic body employed in prior piezoelectric transformers takes various forms and configurations, including rings, flat slabs and the like. A typical example of a prior piezoelectric transformer is illustrated in FIG. 1. This type of piezoelectric transformer is commonly referred to as a "Rosen-type" piezoelectric transformer. The basic Rosen-type piezoelectric transformer was disclosed in U.S. Pat. No. 2,830,274 to Rosen, and numerous variations of this basic apparatus are well known in the prior art. The typical Rosen-type piezoelectric transformer comprises a flat ceramic slab 110 which is appreciably longer than it is wide and substantially wider than thick. As shown in FIG. 1, a piezoelectric body 110 is employed having some portions polarized differently from others. In the case of the prior transformer illustrated in FIG. 1, the piezoelectric body 110 is in the form of a flat slab which is considerably wider than it is thick, and having greater length than width. A substantial portion of the slab 110 the portion 112 to the right of the center of the slab, is polarized longitudinally, whereas the remainder of the slab is polarized transversely to the plane of the face of the slab. In this case the remainder of the slab is actually divided into two portions, one portion 114 being polarized transversely in one direction, and the remainder of the left half of the slab, the portion 116 also being polarized transversely but in the direction opposite to the direction of polarization in the portion 114.
In order that electrical voltages may be related to mechanical stress in the slab 110, electrodes are provided. If desired, there may be a common electrode 118, shown as grounded. For the primary connection and for relating voltage at opposite faces of the transversely polarized portion 114 of the slab 110, there is an electrode 120 opposite the common electrode 118. For relating voltages to stress generated in the longitudinal direction of the slab 110, there is a secondary or high-voltage electrode 122 cooperating with the common electrode 118. The electrode 122 is shown as connected to a terminal 124 of an output load 126 grounded at its opposite end.
In the arrangement illustrated in FIG. 1, a voltage applied between the electrodes 118 and 120 is stepped up to a high voltage between the electrodes 118 and 122 for supplying the load 126 at a much higher voltage than that applied between the electrodes 118 and 120.
An inherent problem of such prior piezoelectric transformers that they have relatively low power transmission capacity. This disadvantage of prior piezoelectric transformers relates to the fact that little or no mechanical advantage is realized between the driver portion of the device and the driven portion of the device, since each is intrinsically a portion of the same electroactive member. This inherently restricts the mechanical energy transmission capability of the device, which, in turn, inherently restricts the electrical power handling capacity of such devices. Additionally, because the piezoelectric voltage transmission function of Rosen-type piezoelectric transformers is accomplished by proportionate changes in the x-y and y-z surface areas (or, in certain embodiments, changes in the x-y and x'-y' surface areas) of the piezoelectric member, which changes are of relatively low magnitude, the power handling capacity of prior circuits using such piezoelectric transformers is inherently low.
Because the typical prior piezoelectric transformer accomplishes the piezoelectric voltage transmission function by proportionate changes in the x-y and y-z surface areas (or, in certain embodiments, changes in the x-y and x'-y' surface areas) of the piezoelectric member, it is generally necessary to alternatingly apply positive and negative voltages across opposing faces of the "driver" portion of the member in order to "push" and "pull", respectively, the member into the desired shape. Prior electrical circuits which incorporate such prior piezoelectric transformers are relatively inefficient because the energy required during the first half-cycle of operation to "push" the piezoelectric member into a first shape is largely lost (i.e. by generating heat) during the "pull" half-cycle of operation. This heat generation corresponds to a lowering of efficiency of the circuit, an increased fire hazard, and/or a reduction in component and circuit reliability. Furthermore, in order to reduce the temperature of such heat generating circuits, the circuit components (typically including switching transistors and other components, as well as the transformer itself) are oversized, which reduces the number of applications in which the circuit can be utilized, and which also increases the cost/price of the circuit.
Another problem with prior piezoelectric transformers is, because the power transmission capacity of such prior piezoelectric transformers is low, it is necessary to combine several such transformers together into a multi-layer "stack" in order to achieve a greater power transmission capacity than would be achievable using one such prior transformer alone. This, of course, increases both the size and the manufacturing cost of the transformer; and the resulting power handling capacity of the "stack" is still limited to the arithmetic sum of the power handling capacity of the individual elements.
Another problem with prior piezoelectric transformers is that they are difficult to manufacture because individual ceramic elements must be polarized at least twice each, and the directions of the polarization must be different from each other.
Another problem with prior piezoelectric transformers is that they are difficult to manufacture because it is necessary to apply electrodes not only to the major faces of the ceramic element, but also to at least one of the minor faces of the ceramic element.
Another problem with prior piezoelectric transformers is that they are difficult to manufacture because, in order to electrically connect the transformer to an electric circuit, it is necessary to attach (i.e. by soldering or otherwise) electrical conductors (e.g. wires) to electrodes on the major faces of the ceramic element as well as on at least one minor face of the ceramic element.
Another problem with prior piezoelectric transformers is that the voltage output of the device is limited by the ability of the ceramic element to undergo deformation without cracking or structurally failing. It is therefore desirable to provide a piezoelectric transformer which is adapted to deform under high voltage conditions without damaging the ceramic element of the device.
It is another problem of prior piezoelectric transformers that they tend to break down (i.e. short) under relatively low voltages.
It is another problem of prior piezoelectric transformers that they do not provide true electrical isolation between the input voltage and the output voltage.
It is another problem with typical magnetic transformers that they are frequency band limited to a bandwidth from 300 Hz to 4,000 Hz. It is therefore desirable to provide a piezoelectric transformer which is adapted to the higher speed data demands of current technology.
It is another problem with prior transformers that, when dealing with the digital circuitry such as ISDN and T1/E1, special transformers must be used that satisfy only the demands of each specific service.
It is another problem with prior transformers that, when dealing with the digital circuitry such as ISDN and T1/E1, separate transformers are necessary for filling the needs of each application frequency bandwidth. This separate need is only satisfied through the use of multiple devices, which is more costly.
Another problem of prior piezoelectric transformers is that the voltage transformation ratio (that is V.sub.out /V.sub.in) is not uniform over wide frequency ranges. Because of this problem, prior piezoelectric transformer applications are typically limited to small frequency ranges (i.e. at or near a natural resonant frequency of the particular device).
Another problem of prior piezoelectric transformers is that because of the problem of non-uniformity of the voltage transformation ratio (V.sub.out /V.sub.in) over wide frequency ranges, prior piezoelectric transformers are not adaptable for simultaneous audio and data signal telecommunications applications requiring wide bandwidth service.
Another problem of prior piezoelectric transformers is that because of the problem of non-uniformity of the voltage transformation ratio (V.sub.out /V.sub.in) over wide frequency ranges, that they require higher input voltages because they do not respond linearly with lower power inputs.
It is another problem with prior transformers that they generate heat, introducing an additional load on the cooling demands of communications equipment.
It is another problem with prior transformers that they generate heat, which introduces noise into telecommunications circuitry.
It is another problem with prior transformers that they cannot withstand heat loads in excess of 600 degrees F., without sustaining damage.
It is another problem with prior transformers that they have low power utilization efficiencies, such as magnetic transformers which have an efficiency loss of up to 40=50%.
It is another problem with prior transformers that in order to handle certain ranges of frequencies, they must have a large size, which is not compatible with inline telecommunications circuitry.
Another problem with prior transformers is that the magnetic core and coiled wire can generate magnetic fields that interfere with surrounding circuitry.
Another problem with prior transformers is that they lack the capability to provide impedance matching with transmission lines which causes line losses and echoes, seriously degrading high-speed data transmission.
Another problem with prior transformers is that they are susceptible to EMF interference
Another problem with prior transformers is that they are difficult to miniaturize for applications within circuit chips.